Japanese Unexamined Patent Application Publication No. Hei 11-233276 has disclosed an illumination control circuit (an illumination adjustment circuit) which drives an illuminating lamp for illuminating car audio device and adjusts its illuminance.
As shown in FIG. 1 of the above-mentioned patent document, the illumination adjustment circuit comprises a resistor (R1) and a zener diode (D1) connected in series with a power supply (Vcc), a switching transistor (Q2) which carries out a switching operation in accordance with a PWM signal (P), a filter circuit formed of resistors (R1, R2) and a capacitor (C1), a driving transistor (Q1) serving as a driving element for supplying a driving electric power from the power supply (Vcc) to an illuminating lamp (FL).
In the illumination adjustment circuit having the above-described structure, the switching transistor (Q2) switches a constant voltage (V2) generated in the zener diode (D1) in response to the PWM signal (P), while a switching output is smoothened by the filter circuit, thereby generating a DC voltage (VB) and determining a potential at the base of the driving transistor (Q1).
In this way, once a pulse width (W) of the PWM signal (P) is adjusted, it is possible to adjust a potential at the base of the driving transistor (Q1) and a driving electric power to be supplied to an illuminating lamp (FL), thus making it possible to adjust an illuminance of the illuminating lamp (FL).
Further, if the pulse width (W) of the PWM signal (P) is maintained at a predetermined width, the potential at the base of the driving transistor (Q1) may be maintained at a constant level by virtue of the DC voltage (VB), so that it is possible to maintain an illuminating lamp (FL) at a constant illuminance.
Patent Document 1: Japanese Unexamined Patent Application Publication No. Hei 11-233276.